Various types of gear devices have been developed in various technical fields such as industrial robots, machine tools, and vehicles. A lubricant (lubricant oil) that reduces friction on a movable part of a gear device is important in prolonging life of the gear device. Japanese Patent Application Publication No. Hei 10-78411 and Japanese Patent Application Publication No. 2005-331324 propose techniques for examining quality of a lubricant. In the conventional examination techniques, a concentration of magnetic powder or an amount of metal powder in a lubricant is detected, and based on a result thereof, quality of the lubricant is evaluated. While being useful for evaluating deterioration in quality of a lubricant over a long period of time, these techniques are not suited for evaluating a malfunction risk of a gear device attributable to a sporadically caused malfunction factor.